Scrasamax:
Doing some basic research on what an Arcology actually is as opposed to what we think an arcology is from TV and video games.

The word Arcology is a portmanteau of the words Architecture and Ecology, and the basic concept is not new, but it is also not quite what comes to mind at first. Rather than giant dystopian megastructures, Arcologies are building practices that reduce man's impact on the enviroment, and initial research shows that these real world arcologies look more like hippie communes with art deco construction techniques. This, while realistic, isn't really fitting into the basic tone and theme of CT. So, i will be sticking with the basic megastructure with a trend towards dystopianism. Mwahahahaha.

The Autonomous BuildingThe basic arcology is going to operate with a large degree of self sufficiency, and with careful use, reclamation and recycling of basic goods, this isn't very difficult. The biggest problem facing the self-sufficient structure is that it cannot really produce the food required to feed its inhabitants. However, this can be averaged out since there is much less land being covered by urban and suburban sprawl that can instead be used for agriculture.

The Luxury of WaterWater is going to be used meagerly to reduce the demand for it. All the plumbing an in arcology is going to be super water efficient, such as limited amounts available for bathing, and toilets are going to be low flow models and perhaps a greater emphasis on a unisex urinal which can operate with little or no water. Hot baths are going to be a luxury for lower class folk, with short showers being more the pace of the day. Unintended consequence can be a resurgence of dry hair shampoo as well as increased use of cologne and perfume. At lower income levels, especially near the bottom, public bath and shower facilities could replace private bathrooms. These spartan conditions are a bit extreme, and would probably not represent a major first rank arcology, but perhaps an older facility, one in a water poor area, or one that is overpopulated.

Water can be collected from roof level cisterns that capture rain water, wells or aqueducts, or from moisture evaporators that literally suck the water out of the ambiet air itself. Any water that can be recycled is going to be, and drinking from a toilet is going to be a bad idea, as there is no reason that you cant use water from a washing machine to flush the crapper (grey water reusage before recycling or purification)

ElectricityRather than being festooned with solar panels and wind mills, the arcology is going to be powered by a Cascade D-Engine (multiple engines linked together through a single control interface). While other resources are limited, the lights are always on, and I imagine an arcology is a sight to behold when the sun goes down.Tangental thoughts: roof top landing pads for VTOLs and moorings for A-Pod equipped ships to tether to the arcology, Zeppelin style. Image: NEG warship linked to an arcology, trading out supplies and restocking, while the crew are granted some shoreleave.

Climate ControlElectric heat, electric AC, the entire megastructure has an artificial environment inside. I can see there being a central climate control center that monitors the humidity, and temperature inside the structure, making adjustments as needed to keep the entire structure close to uniform. With abundant power, this isnt horribly difficult and doesnt have to be super efficient. Of course, if the central climate control goes offline, it is easy for the air in the building to become stale without circulation, and for temps to get out of hand (hot on the sunny side, or freezing temps in colder climates, etc) While not lethal, would certainly be uncomfortable.Tangental thought: The climate and power control centers, the engineering section of the Arcology is going to be a vital part of its operation, and vulnerable to sabotage, or infiltration by blanks, terrorists, or rebels. The central control systems will be guarded, both by para military or police and by some heavy duty computer encryption.

Food ProductionHydroponics would be used to an extent, but currently is is expected that it takes more than 100 square meters of space to grow enough food to feed one average human. Arcological food production is either going to be for core essentials and staples, or I can also see the entire idea being scrapped in favor of ornamental gardens and such so that people have a distraction from the fact that they live their lives in a giant concrete, steel and glass megastructure. With the reduction in both world population, and reduced dependence on suburbia, as mentioned above, more arable soil is being used for farming than for minimalls and subdivisions. These things both still exist, but not at the level and intensity of our modern era. I can see Imminent Domain being used to scrape old communities and their concrete and buildings off of valuable alluvial soil and flood plains to be replaced by NEG combines and farming operations growing bulk crops with Farmville level efficiency.

Anyone who mentions agromechs will be struck about their head and shoulders with a haddock.

Agro-ArcologyDrawn from the wiki article of Vertical Farming, there could be an entire class of arcology that is pretty much a vertical farm. The biggest problem facing the vertical farm idea is that the energy required to light, thermoregulate, and irrigate the structure would outweigh its benefits. Insert the Cascade D-Engine and the power problem is addressed. Wheat and corn, floors 2 through 12, tubers and root crops, floors 13 through 20, and so on. Since it is mentioned that the arcologies can be connected by maglines and other rail or tube based transit systems, the FM Line would be a freight rail that bring food from the Farm to Market.

http://en.wikipedia.org/wiki/Vertical_farming

Communicationswithout the benefit of satellites, communications between arcologies and cities, etc would be done via a mixture of buried fiber optic cables, antenna (either part of the outer shell of the arcology, or sitting on top of it, or insert Arcanotech Handwavium. Ham radio operation could also see a resurgence in popularity

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The Arcology as a Megastructure

An arcology is going to be big, a megastructure. Definition of a megastructure is something more than a kilometer in size, like the Great Wall of China.

So, with this definition the Arcology will need to be about a kilometer tall, which would make a basic model in the 330 story range, and sitting in a typical cityscape, the Arcology is going to be generally 2 to 3 times taller than the surrounding skyscrappers. There are going to be mini-arcologies, such as the Ashford Foundation HQ, which is described as almost being an arcology to itself, which I am imagining as being arcological in function and form, but closer to the size of modern skyscrapers (albeit the larger ones). There are probably also going to be a few arcologies that are more than this baseline size, but they are probably going to be the heart of top ten global population centers.

The Arcologies I am picturing are slab sided beasts like the cube shaped arcology from SimCity2000.

Arcology vs. Domed City

A dome city is a megastructure, but I'm not sure it is what I would first think of for an arcology. The dome fits the basic definition, basicly a large enough construct that operates in a semi-autonomous fashion. The main point is that there is nothing wrong with the air, or outside environment, which is why the domes are generally used. No atmospheric contamination, and a dome is nothing more than scene dressing, a constant requirement for upkeep and heck of a target for the enemies to knock down. Dome cities also sprawl out under a dome, rather than standing tall on a minimal amount of square footing.

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Since the advent of the Arco post-dates the first ArcanoTech War, Arcos are built with an eye towards defense. The entrances to the structure are guarded with scanners and concealed weapons ports, and surface to air missile launchers.

The arcology has in many ways become the modern castle, a fortress for the preservation of humanity. The buildings, by virtue of their size, are capable of absorbing large amounts of damage without suffering structural failure. Damage control teams, engineers, and cadres of soldiers ensure that even damaged, an arco is still a formidable foe.

Limitations:Arcos currently do not mount naval grade weaponry, though the thought of an arco with a rail gun turret on the roof gives me a special feeling. What weapons the structure does mount are going to be weapons with a minimum of recoil. Missiles are popular since they can be used to engage air or surface targets, ships or enemy mecha.

It isnt out of the question to assume that each arco has a miniature military base inside of it. Rather than being a sprawling complex with tanks and aircraft, the 'base' is going to be a HQ for soldiers, power armor troops, and the other defenders. It can also operate as a base of operations for outside military forces, offering supplies, communications, and medical facilities. Most arcos are going to be within close distance of a military base for mecha support. Those that are not can expect the NEG navy to have some sort of presence in the area as deterrent, even if it is just a destroyer or a couple of frigates.

Industrial Arcologies and geocities will without a doubt have heavy support and defence, since their operation is vital to the NEG war effort. I also imagine that most industrial superstructures will be built as geocities to restrict their vulnerability to Migou attack.

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One of the ideas I was wanting to explore was the idea of the arco as a fortress, be it to placate a populace rattled by war, or as a structure to hold a defensive line. This far, no further.

The tech for arcos largely exists now, the biggest limiting factors are structural materials and power supply. if we can build hyperfast mecha the structural issues have been addressed in a fashion not really worth noting. With D-Engines and Cascade D-Engines, power has been made a moot point. Still, distributing power, water, and consumables through an arcology (and air circulation) is going to be a point of vulnerability. One method to address this weakness is to make all the distribution systems in the core of the arco, off limits to residents and only accessable by specially trained personnel. Another is to have a decentralized system, but this makes for much more maintenance, and more places that have to be dedicated to support systems rather than industrial, commercial, or residential ventures. I would have a sort of hybrid of the two, a central system 'the main' that handles most of the distribution, as well as several bypasses routed through the building. The bypasses can help with overloads and high demand, such as a heat wave, or providing power to defensive lasers and weapons in a combat situation.

Can you stealth an arcology?

Something that has to be discussed is the potential for dystopia.

The upper levels and the outer layers of the arco are going to reserved for the wealthy and important, while it is the poor schlubs who have to live with artificial windows cycling through pleasing but fake images of mountains and streams, when there is really hundreds of feet of concrete and steel between them and a single ray of sunlight. Add in filtered air, recycled water, processed food and a limited ability or safety based fear of going outside...

it doesnt take much for the slums, the inner most, deepest levels where the poorest of the poor in the arco live to be forgotten. They might not be poor compared to someone living in a suburb house with intermittent power and rarely running water but they are still forgotten. Feral children running thought corridors that havent seen a new light bulb in years, or a cleaning team in just as long, places where the arco police just have an unspoken blockade.

Almost every complex system has to have a place where the refuse and the waste are collected. Not every person who makes it into the arco is fit to live in the arco and they are eventually going to A. leave B. be arrested and exiled to a prison camp C. end up in the slums of the arco. It doesnt take much for one person to decide that maybe serving an eldritch god cant be much worse than sleeping in a room that has a broken door, no light and reeks of machine oil.

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Arcologies can also be graded.

An A ranked arcology is certainly a gilded cage from which the residents look out across a blasted world and scoff. Such ruin can never come here, we are invulnerable in our fortress of science. Said arcos are the cream of the crop, vigorously defended, well supplied, and well policed.

A B ranked arcology is still pretty nice, but it isnt the guilded cage, there are flaws and so on, but most of the populace is content and it is well maintained and defended.

A C ranked arcology is starting to look like it is in rough shape. It could be overcrowded, suffer from rolling black outs and water shortages, or have organized criminal elements, persistant cult activity, or be located in an unimportant region with minimal defences and supply deliveries are not regular. Still, the lights are on more often than not, and it is safe to go out of your unit, even if there are places to avoid, and places to not go at all after a certain hour.

A D ranked arco is on bad shape. The lights are on less than they are off, water is a valuable commodity and the police could either be repressive or non-existant. Most D ranked arcologies are first generation designs, overpopulated, and bringing them back up to par would be a massive renovation that would only be done after the entire populace is removed.

A F ranked arco is structurally compromised, damaged by battle or natural disaster (pretty much volcanic or earthquake) and the distribution systems are not functional at all in more than half of the arco. Residents are corrupted by criminal activity, drug use, and cult worship. There are not many F ranked arcos as compromising the integrity of a Km tall building means it isnt going to be standing much longer. Structural collapse is not if, but when.

A - 10%B - 35%C - 30%D - 20%F - 5%

Distribution across this curve can be adjusted to fit mood and theme, a brighter shinier happier setting could have more A ranked at the cost of fewer D and C ranks. For a darker setting, A and B could be reduced, thickening the number of C and D ranks. in the above, 75% of Arcos are passing grade, just for perspective.

This is the main idea I am using for a visualhttp://www.danvk.org/wp/wp-content/uploads/2009/06/plymouth.gif

for Geocities

http://en.wikipedia.org/wiki/Underground_city

In general

http://en.wikipedia.org/wiki/Megacity

Pariah:
I'm kinda tired right now, so I just kinda skimmed over this, but here's my ideas on arcos.

Stealth/Defense - Bunker complex, possibly built into a mountain to give some vertical movement in a direction other than down.-I don't know what type of man-portable weapons CT has, but RPG's would definately go a long way as a force multiplier, especially against something like a mech, and definately against something with a lot softer skin, like zepplins.

Windows/Sunlight - If you want the arco to be at least marginally self sufficient, there will need to be a large amount of agriculture going on inside of it, limiting the sunlight to the top of the top as far as incomes go, royalty and/or dirty capitalist pigs, this also has the side effect of providing something to employ your unwashed masses with.

Scrasamax:
Food production and power are the two biggest concerns of the Arco.

CTech handwaves the power concern with D-engines, and food production is dealt with by outside food production, more arable land is being used for farming rather than subdivisions, strip malls, and parking lots.

Other answers include agriculture arcos, vertical farming, where the building is just a series of stacked farms. Power use for pumping water and climate control are currently restrictive, but again, toss in the D-engine tech and the power problem is taken care of. I can see a massive central arco with several smaller green skyscrapers surrounding it for food production.